Rumble eliminator



RUMBLE ELIMINATOR Cosler Donald Knight, Lake Zurich, Ill., assignor to Zenith Radio Corporation, a corporation of Delaware Application November 9, 1954, Serial N 0. 467,799

2 Claims. (Cl. 179--1) This invention relates to audio-frequency signal-translating channels and more particularly to such channels of the type employed in electric phonographs or in wavesignal receivers.

The use of electric phonographs to reproduce intelligence in the audio-frequency range from record discs is widespread. It is highly desirable in such reproduction that the phonograph output be substantially free from extraneous noises, such as rumble or power-line frequency hum, which may be introduced, amplified, and translated to the phonograph speaker. The mechanical parts of a phonograph are never absolutely perfect; even when adhering to minute tolerances, there are imperfections in the drive motor, turntable, and the other parts. Such imperfections can produce an oppressive rumbling noise in the phonograph output, particularly objectionable in high-fidelity instruments during high-frequency or lowvolume passages; power-line frequency hum, when not completely filtered out, may become objectionable during the same passages. In addition some phonographs tend to produce a low-frequency howl which is the result of speaker vibrations being fed back to the phonograph input, which tendency is more pronounced with some pickup structures than with others. The low-frequency range in which these types of noise are found may be entirely eliminated by employing a suitable filter; however, this expedient also results in attenuation of desired low-frequency components and detracts from the fidelity of sound reproduction. In the attempt to eliminate rumble, the mechanical parts of phonographs have been manufactured to tolerances as fine as present-day machine practices permit, yet the problem has not been eliminated. Rumble may even originate in broadcast equipment, be amplified and transmitted to the receivers; thus, objectionable rumble may also be present in radio receivers. Of course, one or more electrical networks may be inserted in the audio-frequency channel to eliminate extraneous rumble, hum and howl. Known networks of this type have been of complex form, often requiring additional vacuum tubes and frequently utilizing feedback paths or comparable circuitry to eliminate undesired noise. In general, the improvement obtained by employing auxiliary noise-reducing tubes and systems is not commensurate with their cost, and very few such arrangements have ever been widely used in commercially produced radios and phonographs.

It is an object of this invention to provide an audiofrequency signal-translating channel which eliminates undesirable extraneous noise, such as rumble, hum, or howl, without substantially detracting from the fidelity of sound reproduction.

It is a further object of this invention to provide such a channel which is structurally simple and mechanically practical for use in quantity production.

In an audio-frequency translating channel constructed in accordance with the invention, an aperiodic frequency- United States Patent selective coupling network is coupled between a source "ice vice. This coupling network presents a substantially higher impedance to signal, components in one frequency range than to signal components in another frequency range; in a preferred embodiment it may simply consist of a low-cut coupling condenser having a substantial impedance in the low-frequency portion of the audio range but having negligible impedance at higher audio frequencies. This frequency-selective coupling network includes at least one non-linear resistor which, because of its non-linear voltage-resistance characteristic, substantially reduces the coupling impedance between the signal source and the sound reproducer only whenever the desired signal components in the one frequency range are of greater strength than undesired components, such as objectionable rumble, hum, or howl, in the same frequency range. The effect of the frequency-selective coupling network is to discriminate against low-frequency noise in the absence of sufficiently strong low-frequency intelligence to mask the noise.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a block diagram, partly in schematic form, of an electric phonograph embodying the invention;

Figure 2 is a graphical representation of an operating characteristic, useful in understanding the operation of the invention; and

Figure 3 is a block diagram, partly in schematic form, of a radio receiver embodying the invention.

In Figure 1 the basic components of an electric phonograph are shown in block form. A conventional phonograph record rests on a turntable 3 which is mechanically connected to a drive motor 2. Drive motor 2 derives its power from an alternating-current source such as a city power supply main (not shown), being coupled to the source through a line cord terminating in a power plug 1. A pickup 4 includes a mechanically intercoupled stylus and transducer assembly; the stylus rests on the record and the transducer is coupled to a phono-preamplifier 5. Phono-preamplifier 5 is coupled through a coupling capacitor 8 to a volume control potentiometer 11 having a movable tap coupled to an audio-frequency amplifier 12, and thence to a sound-reproducing device such. as a loudspeaker 13. In accordance with the invention, a pair of non-linear resistors 9 and 10 are connected in shunt with coupling capacitor 8; non-linear resistors 9 and 10 and coupling capacitor 8 constitute a variable-impedance audio-frequency coupling network 14.

Non-linear resistors 9 and 10 may conveniently take the form of commercially available varistors or Thyrite elements which are cellular in contruction and pass current in only one direction. To provide symmetrical conduction of alternating-current signals, two such non-linear resistors are connected in parallel with opposite polarity senses; the impedance characteristic of each non-linear resistor is shown in'Figure 2, in which resistance is plotted as a function of impressed voltage. For the purpose of the invention, the most useful part of the curve is the initial region from approximately 0 to 0.5 volt, within which resistance variations in the range from one kilohm to one megohm may be achieved. Inspection of this curve shows that within this range a variation of about 0.5 volt in the voltage applied across the non-linear resistance may etfect a resistance variation of as much as 200 times or more.

In operation, the stylus of pickup 4 tracks a record and the transducer converts the mechanical aberrations of the record grooves into electrical signals which are coupled to phono-preamplifier 5. Objectionable rumble may originate in any of the mechanical parts of the drive motor, turntable, or intermediate gearing and has been encountered even in commercially produced highfidelity phonographs. The rumble is a result of lowfrequency vibrations occurring in one of the mechanical parts by reason of a slight unbalance or other minor imperfection in the construction; the majority of objectionable rumble noises originate in the phonograph drive motor. The low-frequency rumble is also transmitted through pickup 4 to phono-preamplifier 5, where it is amplified along with the desired intelligence. The amplified output signal from phono-preamplifier 5, containing desired audio-frequency intelligence and subject to extraneous low-frequency rumble, is coupled through coupling capacitor 8 to volume control potentiometer 11, and thence to audio-amplifier 12 and speaker 13. The capacity of coupling capacitor 8 may be chosen to block or attenuate the low-frequency components so that the undesired rumble is partially or completely suppressed, but this also results in blocking of the desired low-frequency components of the recorded intelligence.

In accordance with the invention, coupling capacitor 8 is proportioned to discriminate against objectionable rumble and hum during high-frequency and low-volume passages; during these passages there is little or no lowfrequency signal applied to coupling capacitor 8, and therefore no appreciable voltage is built up across capacitor 8. At low voltages non-linear resistors 9 and 10 have a very high impedance, of the order of a megohom, and therefore have no substantial effect on the coupling impedance exhibited by network 14. When the strength of the low-frequency components of the intelligence signal from phone-preamplifier 5 exceeds that of the undesired rumble signal, the strong low-frequency signal builds up a voltage across coupling capacitor 8; this voltage also appears across non-linear resistors 9 and 10. As this voltage increases from 0 to about 0.5 volts, the impedance of non-linear resistors 9 and 10 (shown in Figure 2) decreases from about one megohm to one kilohm; because of the shunt connection of non-linear resistors 9 and 10 across coupling capacitor 8, this resistance decrease of variable resistors 9 and 10 substantially lowers the impedance of coupling network 14 to the strong low-frequency signal. The result is that coupling network 14 passes low-frequency signal components when the desired low-frequency intelligence becomes strong enough to mask undesirable rumble. Low-frequency howl originated by acoustic feedback and residual power supply hum are also suppressed in a similar manner. Thus coupling network 14 attenuates objectionable rumble, howl or bum in the absence of desired low-frequency intelligence; only when the strength of the desired low-frequency intelligence components exceeds that of the undesired loW-frequency noise is the impedance of coupling network 14 sufiiciently reduced to pass all of the low frequencies, and under these conditions the rumble, hum and howl are effectively masked or swamped out by the intelligence signal.

It is important that the voltage appearing at the input to coupling network 14 be within the correct range (approximately 0 to 0.5 volt for varistors exhibiting the operating characteristic of Figure 2) to effect proper resistance variations in non-linear resistors 9 and 10 in the presence of strong low-frequency signal components. While this may be assured by expeditious use of signal amplifiers and/or attenuators, it has been found that the correct voltage range is readily obtained with a minimum number of circuit components by employing a barium titanate ceramic transducer in pickup 4. Additionally, coupling network 14 must provide both an input and output impedance match to the associated circuits. This is most easily accomplished by correct choice of circuitry for phono-preamplifier 5 and volume control potentiometer 11.

Figure 3 is a block diagram, partly in schematic form, which shows a radio receiver embodying a coupling network of the type employed in Figure 1. An antenna 24 is coupled to the receiving circuits 25 which may include the customary radio-frequency amplifier, converter, intermediate-frequency amplifier, and audio detector. Receiving circuits 25 are driven from a direct-current power supply 22 which, in turn, is coupled to an alternating-current supply such as the city supply mains (not shown) through a power cord and plug 21. Receiving circuits 2S supply the audio-frequency intelligence signal through a coupling capacitor 28 to the upper terminal of a volume control potentiometer 31 having a movable tap connected to an audio amplifier 32, which in turn is coupled to a loudspeaker 33. A pair of non-linear resistors or varistors 29 and 30 are coupled back-to-back or in opposite polarity in shunt with coupling capacitor 28.

Power supply 22 rectifies and filters the alternatingcurrent supply in a manner well known in the art. Physical limitations on components and economic factors strongly influence the degree of filtering which may practically be employed in such receivers. Usually a slight fluctuation or ripple component appears in the directcurrent output of power supply 22. This ripple is of the the same frequency as the power-line frequency; the ripple may be amplified in the receiver and appear in the output of a non-compensated receiver as an objectionable hum. Coupling network 34, comprising condenser 28 and varistors 29 and 30, discriminates against such low-frequency hum in the same manner that coupling network 14 of Figure 1 discriminates against objectionable rumble in a phonograph.

The invention has used a characteristic of the human ear which reduces its sensitivity to low frequencies at low volume levels to achieve the elimination of rumble noise, power supply hum, and other objectionable lowfrequency noises from an audio-frequency signal-translating channel. Moreover, the coupling network of the invention is a simple one, economical to construct, and highly effective in operation. The improvement provided by the invention is particularly impressive in highfidelity equipment; exhaustive tests have demonstrated that the invention provides virtually complete elimination of rumble, howl and residual hum in such equipment, without any noticeable deterioration in the fidelity or tonal range of the reproduced intelligence.

While particular embodiments of the present invention have been shown and described, it is apparent that various changes and modifications may be made and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Iclaim:

1. An audio-frequency signal-translating channel comprising: a source of audio-frequency signals including desired components in both high-frequency and low-frequency ranges and subject to undesired components in said low-frequency range; aperiodic coupling means consisting of a capacitor presenting a substantially higher impedance to signal components in said low-frequency range than to signal components in said high-frequency range and having One terminal coupled to said source for normally discriminating against signal components in said low-frequency range; means, consisting of a load impedance exhibiting substantially constant impedance throughout the audio-frequency range, connected to the other terminal of said capacitor for translating said audiofrequency signals; means including a nonlinear resistor shunting said coupling capacitor for substantially reducing the coupling impedance between said source and said load impedance only in response to desired signal components in said low-frequency range of greater strength than said undesired components, whereby an output signal, substantially free of said undesired components in the absence of saicl desired components in said low-frequency range and in which said undesired components are substantially masked by said desired lowfrequency components When present, is developed across said load impedance; and means coupled to said load impedance and including a loudspeaker for utilizing said output signal.

2. An audio-frequency signal-translating channel according to claim 1, in which said means shunting said References Cited in the file of this patent UNITED STATES PATENTS 2,210,381 Rechnitzer Aug. 6, 1940 FOREIGN PATENTS 23,162/ 35 Australia June 22, 1936 

